1. Field of the Invention
The present invention generally relates to electro-optical readers, such as laser scanners and imagers and, more particularly, to improved aperture stops and optical components for improving laser intensity modulation over an extended working range or depth of focus in which indicia, such as bar code symbols, are read.
2. Description of the Related Art
Bar code readers are known in the prior art for reading various symbologies such as Universal Product Code (UPC) bar code symbols appearing on a label, or on the surfaces of an article. The bar code symbol itself is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers electro-optically transform the graphic indicia into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like. Readers of this general type have been disclosed, for example, in U.S. Pat. No. 5,600,121, assigned to the same assignee as the instant application, and may employ a portable laser scanning device held by a user, which is configured to allow the user to aim the device and, more particularly, a scanning laser light beam, at a targeted symbol to be read.
The light source in a laser scanning bar code reader is typically a semiconductor laser device. The use of semiconductor devices as the light source is especially desirable because of their small size, low cost and low voltage requirements. The laser beam is optically modified, typically by an optical assembly, to form a beam spot or cross-section of a certain size at a target distance. It is preferred that the cross-section of the beam spot at the target distance be approximately the same as a minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
In moving laser beam readers known in the art, the laser light beam is directed by a lens or other optical components along a light path toward a target that includes the bar code symbol. The moving-beam reader operates by repetitively scanning the light beam in a scan pattern across the symbol by means of motion of a scanning component, such as a moving mirror placed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line, or a series of scan lines, or another pattern, across the symbol, or scan a field of view of the reader, or both.
Bar code readers also include a sensor or photodetector which detects light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. The light is detected and converted into an electrical signal.
Some bar code readers are “retro-reflective”. In a retro-reflective reader, a moving optical element such as a mirror is used to transmit the outgoing beam and receive the reflected light. Non-retro-reflective readers typically employ a moving mirror to transmit the outgoing beam, but have a separate detection system with a wide, static field of view.
Electronic circuitry and software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector is converted by a digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the information or alphanumeric characters so represented. Such signal processors are disclosed in U.S. Pat. No. 5,734,153, assigned to the same assignee as the instant application.
Different symbols have different information densities and contain a different number of elements in a given area representing different amounts of encoded data. The denser the code, the smaller the elements and spacings. Printing of the denser symbols on an appropriate medium is exacting and, thus, is more expensive than printing low density symbols with larger elements. The density of a bar code symbol can be expressed in terms of the minimum bar/space width, also called “module size”, or as a “spatial frequency” of the code, which is in the inverse of twice the bar/space width.
A bar code reader typically will have a specified resolution, often expressed by the module size that is detectable by its effective sensing spot. For example, the beam spot size may be somewhat larger than the minimum width between regions of different light reflectivities, i.e., the bars and spaces of the symbol. The resolution of the reader is established by parameters of the beam source or the detector, by lenses or apertures associated with either the beam source or the detector, by an angle of beam inclination with respect to a plane of the symbol, by a threshold level of the digitizer, by the programming in the decoder, or by a combination of two or more of these factors. The photodetector will effectively average the light scattered from the area of the projected spot which reaches the detector aperture.
The region within which the reader is able to decode a symbol is called the effective working range of the reader. Within this range, the spot size is such as to produce accurate readings of symbols for a given density. The working range is dependent on the focal characteristics of optical components of the reader and on the module size of the symbol.
Many known readers collimate or focus the laser beam using an optical system to create the beam spot of a given size at a prescribed distance. The intensity of the laser beam at this distance, in a plane normal to the beam (ideally approximately parallel to the scanned symbol), is ordinarily characterized by a “Gaussian” distribution with a high central peak. Gaussian beams typically have a profile along their axis of propagation exhibiting a waist (collimated) zone with limited divergence followed by a divergence zone thereafter. The collimated zone determines a depth of field (focusing range) for maximum bar code density. However, as the distance between the reader and the symbol moves out of the working range of the reader, which is typically only a few inches in length, the Gaussian distribution of the beam spot greatly widens, preventing accurate reading of a symbol. Such readers, accordingly, must be positioned within a relatively narrow range of distances from a symbol in order to properly read the symbol.
It has been proposed to modify a laser scanning beam by directing a collimated beam of laser light into an axicon optical element, for example, a conical lens, to produce a beam of light which exhibits a consistent spot size over a substantial distance along an optical axis of the beam. Such an optical system is disclosed in U.S. Pat. No. 5,164,584, U.S. Pat. No. 5,331,143, and U.S. Pat. No. 6,651,888. The conical axicon produces a nearly diffraction-free beam and increases the working range of the scanning beam. Such a beam exhibits substantially no divergence over a relatively long distance range and then breaks into a donut-like spot pattern of intensity distribution. Such a non-diverging beam can provide two to three times the range of a conventional Gaussian beam for a particular symbol density. However, where such a beam is designed to improve performance in scanning a certain bar code density, the corresponding working range of lower density symbols is not increased significantly or at all, being limited by the distance where the beam breaks into a donut-like distribution.
The conical axicon, by itself, produces a generally circular beam spot, which is not desirable for reading one-dimensional UPC symbols where an oval beam spot is preferred because it is less susceptible to errors introduced by voids and ink spreads in the symbol and by speckle noise. Indeed, the narrow dimension of the oval spot is swept along the scan direction to minimize such errors.
Ellipticity of the beam spot can be introduced in an axicon-based reader by employing a diffraction grating. However, there are limitations in the amount of ellipticity that can be introduced, especially as compared to a non-axicon-based optical system in which a conventional Gaussian beam from a laser diode is directed through an aperture.
Also, the conical axicon is sensitive to pointing error of the laser. In other words, fine angular adjustment and alignment between the laser and the axicon are critical for proper operation. Hence, although it is desirable to use a conical axicon to increase the working range, especially in long-range scanners where far-out symbols are located remotely from the reader, the limitations on making the beam spot have an elliptical shape and on rendering the laser source less sensitive to pointing errors tend to prevent the ready adoption of axicons in electro-optical readers.